Method of diffusing a gas turbine compression stage, and diffusion stage for implementing same

ABSTRACT

A diffusion stage of a radial or mixed gas turbine engine compressor includes an impeller formed by two plates, between which fluid flows in a centrifugal or inclined manner from a center towards a periphery. Blades of a cascade are distributed between the plates to channel the flow of the fluid between leading edges of the blades at the center and trailing edges at the periphery. At least one of the plates has an internal face including at least one zone with alternating hollow and bump curvatures between two adjacent blades, in at least one of two substantially perpendicular directions, in the direction of flow along the blades and in an inter-blade tangential direction.

TECHNICAL FIELD

The invention relates to a method for diffusing the flow of air in a compression stage of a gas turbine engine, as well as to a diffusion stage capable of implementing said method.

The field of the invention relates to improving the performance levels and the pumping margin of centrifugal and mixed compressors in the diffusion assembly of the relevant stage. The purpose of this diffusion assembly is to convert the kinetic energy of the fluid, obtained at the output of the centrifugal impeller constituting the stage, into static pressure. The operation must occur with a minimum loss of total pressure whilst maintaining a satisfactory level of stability in the compressor in order to maintain a pumping margin that is acceptable for the operation of the turbine engine.

A centrifugal compressor has at least one radial compression stage, i.e. which is capable of producing a flow of air perpendicular to the central axis of the compressor. A mixed compressor has at least one compression stage that is inclined relative to said central axis.

A diffusion assembly of a compression stage is composed of an impeller formed by two plates, between which the fluid flows in a centrifugal or inclined manner from the centre towards the periphery. Blades are distributed around the impeller between the plates. These blades form a flow cascade between the leading edges of these blades at the centre and the trailing edges on the outside.

PRIOR ART

The plates of the radial and mixed diffusion assemblies are conventionally flat and advantageously the flow cross-sections of the fluid between the blades are tapered. The tapering of the flow cross-sections is defined by the flow cross-section at the neck of the diffuser and by the rate of deceleration between the leading and trailing edges of the cascade.

Other architectures provide axisymmetric plates connected to tapered streams in order to provide additional control of the flow cross-section and to thus optimise the diffusion in the cascade.

These solutions only allow control in one dimension, that of the variation of the flow cross-section. There is no control of the tangential heterogeneity of the flow of the fluid between two blades. However, this control allows the flow to be adjusted and optimised.

DESCRIPTION OF THE INVENTION

The aim of the invention is to produce such a flow by implementing shape-optimised plates as these plates represent the largest surface area that is “streamed” by the flow. Non-axisymmetric shapes in the direction of the flow and in the tangential direction are thus proposed.

More specifically, the present invention relates to a method for diffusing the flow of air in a compression stage of a gas turbine engine comprising a diffusion assembly composed of an impeller formed by two plates, between which the fluid flows in a centrifugal or inclined manner from the centre towards the periphery. Blades of a cascade are distributed around the impeller between the plates so as to channel the flow of the fluid between the leading edges of these blades at the centre and the trailing edges at the periphery. In this method, at least one of the plates has at least one alternation of concave and convex curvatures in at least one of two substantially perpendicular directions, namely in the direction of flow along the blades and in an inter-blade tangential direction.

In these conditions, the three-dimensional shape of the stream of the fluid allows its flow in this stream to be redistributed and homogenised: the secondary flows, which generate load losses, are substantially reduced. The position of shocks in the transonic blade assemblies is modified and their intensity is reduced. Furthermore, the aerodynamic locking at the input of the combustion chamber that follows the compression stage is also substantially reduced.

The invention further relates to a diffusion stage of a radial or mixed gas turbine engine capable of implementing this method. Such a stage comprises an impeller formed by two plates, between which the fluid flows in a centrifugal or inclined manner from the centre towards the periphery. Blades of a cascade are distributed around the impeller between the plates and so as to channel the flow of the fluid between the leading edges of these blades at the centre and the trailing edges at the periphery. At least one of the plates has an internal face comprising at least one zone with alternating hollow and bump curvatures between two adjacent blades, in at least one of two substantially perpendicular directions, namely in the direction of flow along the blades and in an inter-blade tangential direction.

According to advantageous features, the diffusion stage has alternating hollow and bump zones between the blades, in particular up to substantially 80% (preferably up to substantially 50%) of a chord line of a blade, at the leading edge of the blades, starting upstream of the leading edge, and/or at the trailing edge, continuing to downstream of the trailing edge. These alternating hollow and bump zones can be applied to one and/or the other of the two centrifugal (radial) and mixed diffusion plates, particularly in a symmetrical manner, relative to a central plane of symmetry of the plates, or in a parallel manner when the two plates of the stage are involved.

DESCRIPTION OF THE FIGURES

Further information, features and advantages of the present invention will become apparent upon reading the following non-limiting description, with reference to the appended drawings, in which:

FIG. 1 is a partial cross-sectional view of a gas turbine engine comprising an air diffuser;

FIGS. 2 a and 2 b are perspective views of a diffusion stage with blades comprising one and two plates;

FIGS. 3 a to 3 c are schematic views of profiles of a plate in the direction of the air flow along a blade, with two zones with curvatures respectively alternating along the blade, at the trailing edge continuing to downstream of the blade, and at the leading edge from upstream of the blade;

FIG. 4 is a schematic partial view in the inter-blade tangential direction, with a plate having two zones with alternating curvatures;

FIG. 5 is a schematic inter-blade perspective view at the leading edge, with a plate having a zone with alternating curvature; and

FIG. 6 is a schematic perspective view at the trailing edge of two blades, with a flat plate and a plate having a zone with alternating curvature.

DETAILED DESCRIPTION

The terms “downstream” and “upstream” relate to positions in relation to the flow of air. In all of the figures, identical reference numerals relate to the passages in the description in which the elements that correspond to these reference numerals are defined.

With reference to the partial cross-sectional schematic view of a helicopter gas turbine engine 1 according to FIG. 1, an air flow F is firstly aspirated in a fresh air intake sleeve 2, then compressed between the vanes 3 of an impeller 4 of a centrifugal compressor 5 and a casing 10. The turbine has axial symmetry about an axis X′X.

In this case the compressor 5 is centrifugal and the compressed flow F then exits radially from the impeller 4. When the compressor is mixed, the flow exits inclined at an angle of between 0° and 90° relative to a radial direction, perpendicular to the axis X′X.

The flow F then passes through a diffuser or impeller 6, disposed at the output of the compressor 4, in order to be rectified and routed towards intake channels 7 of a combustion chamber 8.

In order to carry out this rectification, the impeller 6 is composed of a plurality of curved blades 60 that are arranged between two plates 9 at the periphery of the impeller 4, in a radial manner in this case, and thus rotate about the axis X′X.

FIG. 2 b more specifically shows a perspective view of the diffuser 6 with blades 60 rigidly connected to two plates 9. In FIG. 2 a, in which a plate has been omitted for the sake of clarity, each blade 60 has, in a known manner, a face 6 e, referred to as an upper face, and a face 6 i, referred to as a lower face. In the example shown, these faces are connected by a tapered leading edge 6 a and a rounded trailing edge 6 f, in the direction of air flow. Transversally to the upper and lower faces, each blade 60 has flat sides 6 p which are rigidly connected to the plates 9.

Conventionally, the plates 9 of FIGS. 2 a and 2 b are flat. According to the invention, at least one of these plates 9 has, in the space E that is defined between them, at least one zone with alternating curvatures between two blades 60.

With reference to FIG. 3 a, such a plate 9 is shown as a profile, in the direction of the air flow F along the blades 60, from the intake at the leading edge 6 a of the blade to the outlet channel 7 towards the combustion chamber. Two zones Z1 and Z2 with alternating curvatures are produced in the plate 9, along the blades 60. Each of these zones has, at the flow F side and in relation to the flat face portion 9 p of the plate 9, a hollow portion 91 and a bump portion 92. The zones Z1 and Z2 generally extend, in the non-limiting example shown, over approximately 80% of the length of the chord line 6 c of the blades 60.

In the figures, the plate 9 has a low thickness in order to simplify the presentation, but in reality it has a certain thickness and the zones with alternating curvatures are formed on the internal face 9 i of the plate where the air flow F flows. The external face 9 e of the plate 9 can remain flat or can also conform to the bump and hollow shapes of the alternating curvatures, which are reversed in this example relative to the hollow and bump shapes of the internal face 9 i. In the first instance, the plate has a variable thickness and, in the second instance, it has a constant thickness. In particular, the shape of the plates can depend on the method used to produce the zones with alternating curvatures: by milling, laser, spark erosion, forming, stamping, etc.

In FIGS. 3 b and 3 c, the two zones Z1 and Z2 with alternating curvatures are respectively formed at the trailing edge 6 f of the blades 60 to downstream of the blades in the intake of the channel 7, and/or at the leading edge 6 a, from the upstream intake of the blades 60.

FIG. 4 shows a partial schematic view of the diffusion stage in the inter-blade “tangential direction” 6 t, i.e. between two blades 60. The arrow F indicates the direction of the flow of air. The internal face 9 i of the plate 9 has two zones Z1 and Z2 with alternating curvatures that mainly extend between the lower 6 i and upper 6 e faces of two blades 60.

The perspective inter-blade view of FIG. 5 at the leading edge 6 a more specifically shows the plate 9 with a zone Z1 with an alternating curvature between the two blades 60. The zone shows the hollow curvature portion 91, on the lower face 6 i of a blade 60, and the bump curvature portion 92, on the upper face 6 e of the other blade 60. This alternation of curvatures allows the pressures between the lower face and the upper face of each blade 60 to be equalised. The section at the neck 6 s, between the leading edges 6 a, is retained.

At the trailing edges 6 f, the homogenisation of the air flow F is shown in perspective view in FIG. 6. With a flat plate 90 (represented by the hatched lines in the figure) between two blades 60, aerodynamic locking is created in the zone Z0 with a very small amount of movement. In this configuration of the prior art, the main air flow (arrow F) has a high Mach number. In contrast, with a plate 9 with alternating curvatures according to the invention, the aerodynamic locking zone is omitted and the flow of air F is homogenised whilst occupying all of the flow cross-section provided, with a lower Mach number.

The invention is not limited to the embodiments described and shown. Therefore, in the examples shown, the curvatures are alternated in the direction of flow of the fluid F but also in the tangential direction 6 t. In other variants, the zones with alternating curvatures can be juxtaposed so that portions of the surface area with the same type of curvature, either hollow or bump, can be close together. 

1.-9. (canceled)
 10. A method of diffusing a flow of air in a compression stage of a gas turbine engine including a diffusion assembly including an impeller formed by two plates, the method comprising: fluid flowing between the two plates in a centrifugal or inclined manner from a center towards a periphery, blades of a cascade being distributed around the impeller between the plates to channel the flow of the fluid between leading edges of the blades at the center and trailing edges at the periphery, wherein at least one of the plates includes at least one alternation of concave and convex curvatures in a direction of flow along the blades.
 11. A method for diffusing flow of air according to claim 10, wherein at least one of the plates also includes at least one alternation of concave and convex curvatures in an inter-blade tangential direction that is substantially perpendicular to the direction of flow along the blades.
 12. A diffusion stage of a radial or mixed gas turbine engine compressor capable of implementing the method according to claim 10, comprising: an impeller formed by two plates, between which the fluid flows in a centrifugal or inclined manner from a center towards a periphery, blades of a cascade being distributed around the impeller between the plates to channel the flow of the fluid between leading edges of the blades at the center and trailing edges at the periphery, wherein at least one of the plates includes an internal face including at least one zone with alternating hollow and bump curvatures between two adjacent blades.
 13. A diffusion stage according to claim 12, wherein at least one of the plates includes an internal face further including at least one zone with alternating hollow and bump curvatures between two adjacent blades in an inter-blade tangential direction.
 14. A diffusion stage according to claim 12, further comprising alternating hollow and bump zones between the blades, up to substantially 80%, or up to substantially 50%, of a chord line of a blade.
 15. A diffusion stage according to claim 13, further comprising hollow and bump zones at the leading edge of the blades, starting upstream of the leading edge.
 16. A diffusion stage according to claim 13, comprising hollow and bump zones at the trailing edge of the blades, continuing to downstream of the trailing edge.
 17. A diffusion stage according to claim 13, wherein the alternating hollow and bump zones are applied to one and/or the other of the two plates for centrifugal and mixed diffusion.
 18. A diffusion stage according to claim 17, wherein the alternating zones, are applied to the plates in a symmetrical manner, relative to a central plane of symmetry of the plates, or in a parallel manner. 